EP1351524B1 - Method for automatic white balance of digital images - Google Patents

Method for automatic white balance of digital images Download PDF

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EP1351524B1
EP1351524B1 EP03075822.1A EP03075822A EP1351524B1 EP 1351524 B1 EP1351524 B1 EP 1351524B1 EP 03075822 A EP03075822 A EP 03075822A EP 1351524 B1 EP1351524 B1 EP 1351524B1
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scene
white balance
value
image
color
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EP1351524A2 (en
EP1351524A3 (en
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Edward Brooks. C/O Eastman Kodak Company Gindele
Jr. James Edward. c/o Eastman Kodak C. Adams
Jr. John Franklin. c/o Eastman Kodak C. Hamilton
Bruce Harold. c/o Eastman Kodak Company Pillman
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Intellectual Ventures Fund 83 LLC
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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N9/00Details of colour television systems
    • H04N9/64Circuits for processing colour signals
    • H04N9/73Circuits for processing colour signals colour balance circuits, e.g. white balance circuits, colour temperature control
    • H04N9/735Circuits for processing colour signals colour balance circuits, e.g. white balance circuits, colour temperature control for picture signal generators

Description

  • The present invention relates to providing for detection of scene illuminant and the use thereof to provide automatic white balance correction in the digital photographic process.
  • To perform like the human visual system, imaging systems must automatically adapt to changing color casts in scene illumination. Simply put, white objects in a scene must be rendered as white, regardless of whether the scene illuminant was daylight, tungsten, fluorescent, or some other source. This process of automatic white adaptation is called "white balancing" and the corrective action determined by this adaptation mechanism is the white balance correction.
  • Automatic white balance algorithms employed in automatic printers, digital scanners, and digital cameras conventionally employ the digitized image information and related mathematical techniques to attempt to deduce from the image data the optimum level of white balance correction to be applied on a scene-by-scene basis to the image. It is known that errors in automatic white balance correction occur when the algorithm is unable to differentiate between an overall color cast caused by the scene illuminant and an overall color bias due to the composition of the scene. It is desirable, therefore, to be able to differentiate a color cast due to scene illumination from a color bias due to scene composition. It is also known that white balance errors occur due to color temperature variations within a class of scene illuminant. Late day direct sunlight imposes a yellowish color cast to a scene while skylight on a cloudy day will lend a bluish color cast to a scene. However, both lights are clearly daylight and will require substantially different white balance corrections. It is desirable, therefore, to also be able to account for scene illuminant color temperature variation when determining the white balance correction.
  • There are many methods described in the literature for determining the scene illuminant of a digital image. Some require special hardware at the time of image capture to make this determination. In commonly-assigned US-A-4,827,119 and US-A-5,037,198 a method of measuring scene illuminant temporal oscillations with the use of a dedicated sensor is described. Daylight will have no oscillation, while tungsten and fluorescent sources will fluctuate in output power due to the AC nature of their power supplies. The problem with any dedicated sensor approach is that it consists of two separate data collection and processing paths, one for illuminant detection and another for actual image capture. This leads to the potential of the dedicated sensor path losing synchronization and calibration with respect to the main image capture path. Additionally, the relatively limited amount of information captured by a dedicated sensor can severely limit the robustness of the scene illuminant determination. In commonly-assigned US-A-5,644,358 and US-A-5,659,357 the image data (video input) is combined with a luminance input to perform illuminant classification. (The nature of the luminance input is never described.) Rather than determining an overall illuminant for the scene, a low resolution version of the image is created and each image element (or "paxel") within the low resolution image is individually classified into one of a number of possible scene illuminants. Statistics are performed on these paxel classifications to derive a best compromise white balance correction. The problem with this approach is that no explicit attempt is made to uncouple the effects of scene illuminant color cast from the effects of scene composition. Instead, a complex series of tests and data weighting schemes are applied after the paxel classifications to try and reduce subsequent algorithm errors. Japanese Patent JP2001211458 teaches a method very similar to that described in commonly-assigned US-A-5,644,358 and US-A-5,659,357 , and has the same problems.
  • There are many methods described in the literature for determining a color temperature responsive white balance correction of a digital image. In commonly-assigned US-A-5,185,658 and US-A-5,298,980 a method of measuring the scene illuminant's relative amounts of red (R), green (G), and blue (B) power with dedicated sensors is described. The white balance correction values are derived from the ratios of R/G and B/G which are considered to be related to the color temperature of the scene illuminant. As with commonly-assigned US-A-4,827,119 and US-A-5,037,198 , discussed above, the problem with any dedicated sensor approach is that it consists of two separate data collection and processing paths, one for illuminant detection and another for actual image capture, and these two paths can get "out of step" with each other. In the above referenced JP200121458 the illuminant classification step is further refined to represent a variety of subcategories within each illuminant class. In this way cooler and warmer color cast versions of the illuminant classes of daylight, tungsten, and fluorescent are determined. However, as stated before, there is no explicit method given for uncoupling illuminant color cast from scene composition variability and, as a result, a variety of involved statistical operations are required in an attempt to minimize algorithmic errors.
  • In commonly-assigned US-A-6,133,983 Wheeler discloses a method for optical printing of setting the degree of color correction; that is, a parameter used to determine the magnitude of applied color balancing to photographic images, based on camera meta-data. In particular, Wheeler discloses using the scene-specific measurements of the scene light level, camera-to-subject distance, flash fire signal, and flash return signal to classify an image as being captured either under daylight or non-daylight illuminant. It is stated that for images captured with daylight-balanced films there is no need to further distinguish the non-daylight illuminants because the same white balance correction methodology works regardless. As a result, commonly assigned US-A-6,133,983 does not present a method for such subsequent illuminant discrimination. This approach fails when applied to imaging systems requiring further differentiation of non-daylight sources for accurate white balancing, or if any of the image metadata (that is, scene light level, camera-to-subject distance, flash fire signal, and flash return signal) are corrupt or missing. In particular, the method disclosed by Wheeler requires the scene light level to be a measured quantity.
  • In conference paper "Usage of DSC meta tags in a general automatic image enhancement system" from the Proceedings of SPIE Vol. #4669, Sensors and Camera Systems for Scientific, Industrial, and Digital Photography Applications III, January 21,2002, the authors Moser and Schroder describes a method of scene analysis regarding the likelihood of a photographic scene having been influenced by an artificial illuminant light source. The method disclosed by Moser and Schroder uses the camera meta-data of Fnumber (f) and exposure time (t) to calculate a "pseudo" energy (pE) quantity for a digital image derived from a digital camera using the formula: pE = ln t / f 2 .
    Figure imgb0001
    Moser and Schroder then use the pE quantity to analyze digital images with regard to the likelihood of the scene illumination source and corresponding resultant color cast. While the method disclosed by Moser and Schroder is useful for analyzing digital images, as disclosed it is not accurate enough to produce consistent automatic white balance correction results for a practical digital enhancement system. This is principally due to the inherent relative, as opposed to absolute, nature of the "pseudo" energy quantity. Two digital cameras with substantially different energy requirements for producing acceptable images will have substantially different "pseudo" energy values for the same scene illumination conditions. Similarly, these same two digital cameras can produce identical "pseudo" energy values when producing digital images with substantially different scene illumination sources.
  • US-A-5,289,268 Patent discloses a white balance adjuster of an electronic still camera for recording a still image. Scene illuminants are determined and subsequent white balance correction is adjusted, based on the difference between the overall scene brightness, information which is the output of a photometering unit, and a green component brightness converted value produced by a color metering unit.
  • US 2001/0043277 A1 Patent Application discloses a method of optimizing a display mode in a display, such as an electronic viewfinder or a liquid crystal display in an electronic camera. This method enables to modify the brightness, the contrast, and the white balance (hue) correction of the electronic viewfinder responsive to the overall scene brightness (indicative of the brightness of a subject) and the results of a white balance circuit which converts the level of pixel data of each of the color components R, G, and B of the subject. The overall brightness is computed from an equation using the camera settings at the time of capture.
  • It is therefore the object of the present invention to provide a simplified way for detecting and discriminating scene illuminants for use in computing white balance corrections and for the white balance corrections to be responsive to color temperature variations within each class of scene illuminants.
  • It is a further object to provide an improved way of providing white balance corrections in a digital image.
  • These objects are achieved in a method of adjusting the color characteristics of a digital image as defined in claim 1. Specific embodiments of the invention are defined in the dependent claims.
  • It is an advantage of the present invention to provide a way of detecting and discriminating scene illuminants to effectively make white balance corrections.
  • Another advantage of the present invention is to provide an improved and simplified arrangement for providing white balance corrections.
    • FIG. 1 is a block diagram of an electronic camera that can be used in accordance with the present invention;
    • FIG. 2 shows in more detail a block diagram of block 22 in FIG. 3;
    • FIG. 3 shows in more detail a block diagram of block 32 in FIG. 2;
    • FIG. 4 shows in more detail a block diagram of block 42 in FIG. 3; and
    • FIG. 5 shows in more detail a block diagram of block 46 in FIG. 3.
  • Since electronic cameras are well known, the present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus and methods in accordance with the present invention. Elements not specifically shown or described herein can be selected from those known in the art.
  • Referring initially to FIG. 1, an electronic still camera is divided generally into an input section 2 and an interpolation and recording section 4. It will be understood that the present invention can be used to process digital images derived form other image capture devices. The input section 2 includes an exposure section 10 for directing image light from a subject (not shown) toward an image sensor 12. Although not shown, the exposure section 10 includes conventional optics for directing the image light through a diaphragm, which regulates the optical aperture, and shutter, which regulates exposure time. The image sensor 12, which includes a two-dimensional array of photosites corresponding to picture taking elements of the image, is a conventional charge-coupled device (CCD) or complementary metal-oxide silicon (CMOS) sensor.
  • The present invention begins by acquiring linear red, green, and blue (RGB) image data from the image sensor 12. The data can be a complete RGB image, or it can be data from a color filter array (CFA) 13, known as the Bayer array, which is described in commonly-assigned US-A-3,971,065 the disclosure of which is incorporated herein by reference. In the Bayer geometry each color covers a photosite, or picture element (pixel), of the image sensor 12. The image sensor 12 is exposed to image light so that analog image charge information is generated in respective photosites. The charge information is applied to an output diode 14, which converts the charge information to analog image signals corresponding to respective picture elements. The analog image signals are applied to an A/D converter 16, which generates a digital image signal from the analog image signal for each picture element. The digital signals are applied to an image buffer 18, which can be a random access memory (RAM) with storage capacity for a plurality of images.
  • A control processor 20 generally controls the input section 2 of the camera by initiating and controlling exposure (by operation of the diaphragm and shutter (not shown) in the exposure section 10), by generating the horizontal and vertical signals required to access the image data in the image sensor 12, and by enabling the A/D converter 16 in conjunction with the image buffer 18 for each signal segment relating to a picture element. Once a certain number of digital image signals have been accumulated in the image buffer 18, the stored signals are applied to a digital signal processor 22, which controls the throughput processing rate for the interpolation and recording section 4 of the camera. In addition, the control processor 20 transmits photographic information to the digital signal processor 22, including camera settings such as shutter speed, aperture, and exposure index. The digital signal processor 22 applies the appropriate image processing algorithms (such as white balance, interpolation, and color correction) to the digital image signals and sends the image signals to a conventional, removable memory card 24 via connector 26.
  • Since image processing ordinarily occurs over several steps, the intermediate products of the processing algorithm are stored in a processing buffer 28. (The processing buffer 28 can also be configured as part of the memory space of the image buffer 18.) The number of image signals needed in the image buffer 18 before digital processing can begin depends on the type of processing, that is, for the white balance operation to begin, a block of signals including at least a portion of the image signals comprising a video frame must be available. Consequently, in most circumstances, the white balance operation can commence as soon as the requisite block of picture elements is present in the image buffer 18.
  • An operation display panel 30 is connected to the control processor 20 for displaying information useful in the operation of the camera. Such information might include typical photographic data, such as shutter speed, aperture, exposure bias, and so on. Moreover, other information unique to this type of camera is displayed. For instance, the removable memory card 24 would ordinarily include a directory signifying the beginning and ending of each stored image. This would show on the display panel 30 as either (or both) the number of images stored or the number of image spaces remaining, or estimated to be remaining.
  • Referring to FIG. 2, a digital signal processor block 22 is described in greater detail. A white balance block 32 accepts unprocessed image data from the image buffer block 18 and classifies the scene illuminant (the light source in use when the image was captured) and determines the proper white balance gains, that is, the white balance correction. In digital camera systems, it is common practice that neutral colors (that is, white, black, and shades of gray) be represented by red, green, and blue (RGB) data values that are equal to each other. When this condition holds, the image is said to be white balanced. However, it is typical that (raw) unprocessed image data from a camera sensor is not white balanced, thus the red, green, and blue data values for neutral colors are not equal to each other. In this case, white balance is achieved by multiplying the RGB data values by different numbers: the red values by a red gain, the green values by a green gain, and the blue values by a blue gain. If the gains are selected properly, the result is that neutral colors have equal RGB values and white balance is achieved. A color interpolation block 34 creates a full three-color image from the white balanced image data created in block 32. Methods of color interpolation are well known (see, for example, commonly-assigned US-A-5,652,621 ) for such color filter arrays as the Bayer pattern (commonly-assigned US-A-3,971,065 ). A color correction block 36 adjusts the RGB data values by the application of a color correction matrix which is selected according to the scene illuminant classification as determined in the white balance block 32.
  • Referring to FIG. 3, the white balance block 32 is described in greater detail. A compute avelll block 42 accepts unprocessed image data from the image buffer block 18 and computes a color coordinate value, called avelll, by an averaging procedure as described below. A compute Bv block 44 accepts photographic information from the control processor 20 and determines the scene brightness value, called Bv, as described below. A determine scene illuminant block 46 accepts scene color information from the compute aveIll block 42 and scene brightness information from the compute Bv block 44. The scene illuminant is classified as one of a plurality of predefined illuminant types.
    A compute white balance block 48 accepts the illuminant classification type and determines the white balance gains. The final step is to apply the appropriate white balance gains to the linear RGB image data.
  • - Referring to FIG. 4, the compute aveIll block 42 is described in greater detail. A paxelize RGB image data block 52 creates a low resolution version of the image by averaging the available red, green, and blue values in an array of pixel blocks, called paxels. The result is a 24x36 array of paxels which cover the central portion of the original image, such that each paxel has an average red, green, and blue value. The paxelized RGB averages are then passed to a convert to log U-space block 54 in which a sequence of transformations is applied. For each paxel, the linear RGB data values (linR, linG, linB) are converted to log RGB values. In the case of 12-bit linear data, which covers the range from 0 to 4095, the log RGB values are computed as follows: logR = Round 1000 * LOG 10 1 + 1 inR
    Figure imgb0002
    logG = Round 1000 * LOG 10 1 + 1 inG
    Figure imgb0003
    logB = Round 1000 * LOG 10 1 + 1 inB
    Figure imgb0004
    yielding another triplet of 12-bit integers. The log RGB values are then converted into log U-space color component coordinates as follows: NEU = logR + logG + logB / 3
    Figure imgb0005
    GM = logR + 2 * logG logB / 4
    Figure imgb0006
    ILL = logR + logB / 2
    Figure imgb0007
    The result is a log U-space triplet (NEU, GM, ILL) for each paxel thus establishing a luminance-chrominance representation of the original red, green, and blue values.
  • The next value to be computed is called sbaNeu18 which is an estimate of the NEU value of an 18% gray patch if one were present in the image. The value sbaNeu18 is determined (in block 56) from two intermediate neutral values which are now described in detail. To find the first intermediate value, the 24x36 paxelized image is divided into four horizontal strips of six rows each. In each horizontal strip the maximum NEU value (MN) is found and noted. Labeling the MN values 1 to 4 from top to bottom, we can express a composite average, called HWA (horizontal weighted average) as follows: HWA = 1 * NM 1 + 4 * NM 2 + 4 * NM 3 + 3 * NM 4 / 12
    Figure imgb0008
  • To find the second intermediate value, the original 24x36 array of paxels is used to define edge paxels. For each paxel having a 3x3 neighborhood of paxels, find the maximum NEU value (MAX) and the minimum NEU value (MIN). If (MAX - MIN) exceeds a specified threshold value (for example, the value 240 works well) the paxel is called an edge paxel. Each edge paxel receives a weight wgt(i,j) (from a gaussian distribution) depending on its location: wgt i j = 1 + 3 * exp 1 2 x j 2 σ x 2 + y j 2 σ y 2
    Figure imgb0009
    y i = i 3 5 * Nrow
    Figure imgb0010
    where i and j are the indices of the paxel's location in the array and where x j = j Ncol 1 2
    Figure imgb0011
    and
    and where Ncol is 36 and Nrow is 24. σ x = Ncol 4 σ y = Nrow 4
    Figure imgb0012
    The weighted average of the NEU values from edge paxels is called GWA (Gaussian weighted average). Using the values HWA and GWA, sbaNeu18 can be computed: sbaNeu 18 = 0.35 * HWA + 0.65 * GWA 0.3324
    Figure imgb0013
  • A select useful paxels block 58 accepts the paxelized image and the sbaNeu18 value from which it determines which paxels are useful to classifying the scene illuminant. Any paxel having a NEU value between sbaNeu18 and (sbaNeu18 + 700) is considered useful. These paxels have an estimated average scene reflectance falling in the range between 18% and 90%. In block 60, the average value aveIll is found by averaging the ILL coordinate of those paxels identified as useful in block 58.
  • Referring to block 44 in FIG. 3, photographic information is used to compute the scene brightness value Bv . Specifically, the photographic data is the aperture setting (f#), the shutter time (t), and the exposure index (ISO). From these three values, Bv can be computed as follows: B V = T V + A V S V
    Figure imgb0014
    where T V = log 2 1 t
    Figure imgb0015
    S V = log 2 ISO π
    Figure imgb0016
    and A V = log 2 f# 2
    Figure imgb0017
  • Using the two values, Bv and aveIll, as inputs, block 46 classifies the scene illuminant as one of a plurality of illuminant types. The preferred embodiment uses just three types, daylight, fluorescent, and tungsten. Referring to FIG. 5, the determine scene illuminant block is described in greater detail. A decision block 62 tests to see if Bv is greater than a threshold Bday (a typical value of Bday is 5). If the inequality of block 62 is true (T), the scene illuminant is classified as daylight in block 70. If the inequality is false (F), an illuminant score (Z) is computed in block 64 using the two values Bv and aveIll as follows: Z = aveI 11 + 25 * B v
    Figure imgb0018
    A decision block 66 tests to see if Z is greater than a threshold Zday (a typical value for Zday is 20). If the inequality of block 66 is true, the scene illuminant is classified as daylight in block 70. If the inequality is false, a decision block 68 tests to see if Z is greater than a threshold Zfluor (a typical value for Zfluor is-170). If the inequality of block 68 is true, the scene illuminant is classified as fluorescent in block 72. If the inequality is false, the scene illuminant is classified as tungsten in block 74.
  • Referring to FIG. 3, the compute white balance block 48 determines the proper RGB white balance gains, as a function of the scene illuminant classification, and applies those gains to the unprocessed image data received from image buffer block 18. The determination of correct white balance gains for each illuminant category is an empirical process. Beginning with the collection of a large number of images that contain an appropriate color test chart, including specifically an 18% reflectance gray patch, one manually determines the color channel gains needed to achieve a perfect white balance correct of the 18% gray patch. These color channel gains are then used to determine, by conventional least-squares function fitting, the coefficients in the following two white balance correction curves: ILLwb = B 1 * B v + B 2 * aveI 11 + B 3
    Figure imgb0019
    GMwb = C 1 * B v + C 2 * aveI 11 + C 3
    Figure imgb0020
    where ILLwb and GMwb are the computed U-space color chromaticity coordinates corresponding to the 18% gray patch. The values ILLwb and GMwb can then be used to determine the corresponding white balance gains as disclosed in commonly-assigned US-A-6,243,133 (Spaulding and others). Depending on the particular camera system, the least-squares methodology can indicate that a single value of ILLwb and of GMwb is adequate to represent the entire illuminant category (that is, B1 = 0, B2 = 0, C1 = 0, and C2 = 0). In other cases, all coefficients are required to be non-zero.
  • An important aspect of the present invention relates to choice of mathematical formula used to calculate color component coordinates and therefore the color coordinate value from the original red, green, and blue pixel values. In particular, the formulation of the ILL term as a difference of the blue and red pixel values is important. Natural scene illumination sources vary in magnitude along a chrominance axis. For example, the ILL color component coordinate, relating to a gray reflective object, varies widely for overcast (slightly blue), daylight (color-neutral), late day and early day sun (golden), and sunset (intense red) illumination sources. These same illumination source have very small magnitude GM color component coordinate values. Thus the choice of mathematical formulation for the color component coordinate (ILL) for basing the calculation of color coordinate value is important since the ILL color component coordinate inherently relates to the natural variation for scene illumination sources. These natural scene illumination source follow a chrominance axis pertaining to a hue line that approximates a blackbody radiation source. Those skilled in the art will recognize that while the choice of the mathematical formulation for the color component axes does not exactly follow a hue line of blackbody radiation sources, the GM and ILL formulation is approximately correct. It will also be appreciated that there are other choices for the mathematical formulation of the color component axes that are also approximate the hue line of a blackbody radiation sources and are considered essentially the same as the choice of the preferred embodiment.
  • While the above discussion describes the present invention as a component of a digital camera, the present invention also can be implemented as a component of a digital photofinishing system. In a digital photofinishing system digital images can be received from a variety of sources including digital cameras and photographic film scanners. The system processes the digital images to adjust the color characteristics of the digital images and produces photographic prints using a digital color printer. In this implementation of the present invention, the uncorrected color digital image pixel values are received by the digital photofinishing system and the white balance correction procedure as described above is performed by the digital photofinishing system. Thus performing a white balance correction, sometimes referred to as color balance, is a form of adjusting the color characteristics of a digital image.
  • An important aspect of the present invention is the determination of the white balance correction as derived from camera meta-data, that is, non-pixel data recorded by the camera and passed along with a digital image. In particular, the camera meta-data used relates to camera capture parameters specific to the digital image. The aperture setting is received as meta-data along with the digital image in the form of an aperture value Av as described above. Similarly, the shutter time is received as meta-data in the form of a time value Tv . The exposure index (ISO) is also received as meta-data in the form of a speed value Sv. As described above, the scene brightness value Bv is calculated from the quantities Av, Tv, and Sv. Thus the digital photofinishing system adjusts the color characteristics of the received digital images using the aperture value, time value, and speed value recorded by the camera.
  • In a practical digital photofinishing system, some received digital images have no accompanying meta-data, while other digital images have the requisite meta-data quantities Av, Tv, and Sv, and still other digital images have a meta-data relating to a measured scene brightness value Bv derived from a photoelectric device within the camera. It is important to note that the quantities Av, Tv, and Sv are used to produce a calculated scene brightness value, that is, a surrogate for a measured scene brightness value Bv . In general, a measured scene brightness value is more reliable than the calculated scene brightness value since a measured quantity directly relates to the ambient illumination intensity. The ambient illumination intensity is used infer the different types of scene illumination sources which have a different characteristic color. The calculated scene brightness value, as used by the present invention, is nearly equivalent to a measured scene brightness value when no electronic flash was used. When electronic flash was used by the camera, the calculated scene brightness value can differ substantially from the corresponding measured scene brightness value. However, for the case when electronic flash was used, the calculated scene brightness value produced corresponds more nearly to a value that would be produced with a daylight illumination source - a substantially elevated value relative to the brightness of the actual ambient illumination source. The present invention makes implicit use of the fact that daylight and electronic flash illumination sources are similar in color. Since the present invention is mainly concerned with correcting white balance, the fact that electronic flash illumination photography can result in an elevated value for the calculated scene brightness value does not significantly affect the accuracy of the achievable white balance correction.
  • When camera meta-data relating to electronic flash illumination conditions and a measured scene brightness value Bv are available, the digital photofinishing system preferentially uses the measured scene brightness value Bv instead of the calculated scene brightness value. If the meta-data indicates that electronic flash was used to derive the digital image, the present invention does not apply the above mentioned white balance correction. If the meta-data indicates that electronic flash was not used to derive the digital image, the present invention applies the above mentioned white balance correction and uses the measured scene brightness value. Therefore the digital photofinishing system uses the calculated scene brightness value Bv as derived from the meta-data quantities Av, Tv, and Sv , when a measured Bv is not available, that is, when no measured scene brightness value has been received.
  • It should be noted that for the purposes of the present invention the term aperture value refers to any meta-data from which the aperture setting of the camera can be derived. Similarly, the term time value refers to any meta-data from which the shutter speed, mechanical or equivalent electrical shutter speed, of the camera can be derived. The term speed value refers to any meta-data from which the exposure index of the camera can be derived.
  • In a practical digital photofinishing system, digital images can be received from a variety of different types of digital cameras with a variety of different pieces of meta-data. In particular, some digital cameras have already performed significant white balance correction on the digital image data. For some digital images received by the digital photofinishing system a pre-balance meta-data tag indicating if the digital image has been previously corrected for white balance relating to an artificial scene illumination source is also received with the digital image pixel data. When a pre-balance meta-data tag is received and indicates that the corresponding digital image has been previously corrected for white balance relating to an artificial scene illumination source, the present invention does not apply the above mentioned white balance correction. When a pre-balance meta-data tag is received and indicates that the corresponding digital image has not been previously corrected for white balance relating to an artificial scene illumination source, the present invention applies the above mentioned white balance correction.
  • The use of the present invention can be detected as a component of a competitor's product such as a digital photofinishing system or a digital camera. In a digital photofinishing system, a set of test digital images is acquired with a digital camera with natural scenes that vary as to scene illumination sources, that is, daylight, late day sun, and overcast. The digital camera if configured such that a white balance correction is not applied to the digital images and does record the meta-data quantities Av, Tv, and Sv as described above. Next, a copy of the set of test digital images are made, that is exact duplicates with regard to pixel values. Next the meta-data quantities Av , Tv, and Sv are deleted from the digital image files for the copy of the test images. The original set of test digital images and the meta-data altered set of test digital images are transmitted to the competitor's digital photofinishing system whereby photographic prints are generated from the two sets of digital images. Since the pixel data of these two sets of digital images are identical, any variation in the color characteristics of the generated photographic prints must be attributable to the camera meta-data. In similar fashion, a test can also be conducted to determine if there is a dependence on any unique combination of the meta-data quantities described above by deleting selected meta-data items. In particular, the scene brightness value Bv meta-data can be inserted in the digital image files to determine if the resulting photographic prints show a color dependency.
  • The likelihood of the use of the present invention as a component of a competitor's digital camera can be determined. A studio test scene is constructed using a tungsten illumination source capable of generating a controlled scene brightness value. Two digital images are produced with the digital camera in question - a first digital image produced with a low ambient illumination level with the digital camera configured not to use the electronic flash and a second digital image produced with a higher ambient illumination level with the digital camera such that the digital camera does not automatically use the electronic flash and a neutral density filter applied over the camera lens. The digital camera is configured to use the same Fnumber and exposure time for both digital images. The second digital image will have essentially the same starting pixel values as the first digital image prior to the application of an auto white balance correction algorithm due to the combination of the neutral density filter and the higher level of ambient illumination. Thus the calculated scene brightness values for the two digital images should be essentially the same while the measured scene brightness values should be different. If the first and second digital images show no difference in color cast when viewed on a display device then it is very unlikely that the present invention is employed by the digital camera being tested. If however, the first digital image shows significant correction for white balance and the second digital image does not, it is likely that the present invention also has been implemented in the digital camera since the only difference in the first and second digital images is the measured scene brightness value. If both or neither of the digital images show a significant and consistent correction for white balance the it is not likely that present invention is implemented in the digital camera in question. This test must be repeated with different absolute ambient illumination levels to determine if a positive indication results.
  • A computer program product may include one or more storage medium, for example; magnetic storage media such as magnetic disk (such as a floppy disk) or magnetic tape; optical storage media such as optical disk, optical tape, or machine readable bar code; solid-state electronic storage devices such as random access memory (RAM), or read-only memory (ROM); or any other physical device or media employed to store a computer program having instructions for controlling one or more computers to practice the method according to the present invention.
  • Other features of the invention are included below.
  • The method where the color coordinate value is defined by ILL = B R 2
    Figure imgb0021
    where ILL is the color coordinate value, R is red and B is blue.
  • The method where the white balance parameter value is defined by s = A 1 × B V + A 2 × aveill + A 3
    Figure imgb0022
    • where Bv is the scene brightness value, aveill is a color coordinate value, and A1, A2, and
    • A3 are coefficient responsive to the scene illuminant type:
  • The method where the white balance correction curve is defined by c = B 1 × B V + B 2 × aveill + B 3
    Figure imgb0023

    where aveill is a color coordinate value, B1, B2, and B3 are coefficients responsive to the scene illuminant type.
  • The method wherein the digital image is derived from a digital camera.

Claims (9)

  1. A method of determining white balance gains comprising:
    (a) receiving a digital image including a plurality of pixels (18);
    (b) receiving non-pixel data relating to camera capture parameters including an aperture value, an exposure time value, and an exposure ISO index in the form of a speed value specific to the received digital image (20);
    (c) using the aperture value, time value, and the exposure ISO index to calculate a scene brightness value (44); and
    (d) using the calculated scene brightness value to adjust the color of the digital image (32) by:
    (i) using the image pixel data to compute a color coordinate value (42);
    (ii) determining a scene illuminant type from the computed scene color coordinate value and the calculated scene brightness value (46);
    (iii) using the determined scene illuminant type to determine white balance gains (48).
  2. The method of claim 1, further including using one or more color coordinate values to determine the scene illuminant type (46).
  3. The method of claim 2, further including the steps of:
    determining a white balance parameter value responsive to the scene illuminant type, the calculated scene brightness value, and the color coordinate values;
    providing at least one white balance correction curve;
    determining the white balance correction from the white balance parameter value and at least one white balance correction curve for the determined scene illuminant type (48); and
    using the white balance correction to adjust the color of the digital-image.
  4. The method of claim 1, wherein the scene illuminant types include daylight, flash, tungsten, and fluorescent (46).
  5. The method of claim 2, wherein the color coordinate value relates to a chrominance axis pertaining to a hue line that approximates a blackbody radiation source (42).
  6. The method of claim 2, wherein the color coordinate value is defined by ILL = B R 2
    Figure imgb0024
    where ILL is the color coordinate value, R is red and B is blue (42).
  7. The method of claim 3, where the white balance correction curve is defined by C = B 1 × B V + B 2 × aveill + B 3 ,
    Figure imgb0025
    where aveill is a color coordinate value, B1, B2, and B3 are coefficients responsive to the scene illuminant type (48).
  8. The method of claim 1, further including providing at least one white balance correction curve for each scene illuminant type (48).
  9. The method of claim 8, where the scene illuminant types includes daylight, flash, tungsten, and fluorescent (46).
EP03075822.1A 2002-04-04 2003-03-24 Method for automatic white balance of digital images Active EP1351524B1 (en)

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